1. Field of the Invention
The present invention relates to a process for the preparation of blocked polyisocyanates containing isocyanurate groups by conducting the blocking reaction prior to the trimerization reaction.
2. Description of the Prior Art
It is known to block isocyanurate group-containing polyisocyanates for subsequent use in powder coatings or solvent-containing stoving lacquers. When blended with a suitable isocyanate-reactive component and heated to an elevated temperature sufficient to release the blocking agents, the polyisocyanates react with the isocyanate-reactive component, generally a polyol, to form a coating.
One of the disadvantages of conducting the trimerization reaction before the blocking reaction is that it is difficult to obtain the same end point for the trimerization reaction. To even approximate the same end point it is necessary to constantly monitor the isocyanate content and, when the desired content is reached, to terminate the reaction by heating and/or by the addition of a catalyst poison.
A more advantageous method for preparing the blocked polyisocyanates would be to conduct the blocking reaction before the trimerization reaction. The amount of the blocking agent can be precisely added and after the blocking reaction the remaining isocyanate groups can be trimerized. Since the amount of isocyanate groups remaining after the blocking reaction can be accurately determined, identical blocked polyisocyanates can be produced from batch to batch. This is advantageous because it ensures consistent product quality.
However, when attempts were made to prepare blocked polyisocyanates by conducting the blocking reaction prior to the trimerization reaction using common blocking agents, such as methyl ethyl ketoxime and .epsilon.-caprolactam, either the trimerization reaction did not proceed or was only partially complete when using standard trimerization catalysts, or significant amounts of by-products were obtained when additional catalyst was added to reduce the isocyanate content.
Accordingly, it is an object of the present invention to prepare blocked polyisocyanates in which the blocking reaction is conducted before the trimerization reaction and in which the trimerization can be conducted to completion or substantially to completion without the formation of significant quantities of by-products.
This object may be achieved in accordance with the present invention by using certain blocking agents for the blocking reaction and by conducting the trimerization reaction using certain trimerization catalysts to be described hereinafter.